State-of-the-art position determining systems need to provide rapid and accurate three-dimensional position determination of mobile, handheld, or portable devices. For example, the FCC has mandated that when a mobile telephone user makes an emergency call in the U.S., the service provider must be able to locate the user to within 50 meters within 30 seconds. This service is known as enhanced 911 (E911).
Among techniques employed to determine the position of a mobile device is the reception at the mobile device of multiple timing signals respectively transmitted from multiple transmitters at different, known locations. For example, the timing signals can be global positioning system (GPS) signals transmitted by satellites or signals transmitted from terrestrial sources such as cellular telephone towers. By determining the range or pseudo-range to each transmitter from the arrival time of the timing signals, the mobile device can compute its position using trilateration or, more generally, N-lateration, where N is the number of timing signal sources, which can be three or more.
One shortcoming of current N-lateration algorithms is the inability to adequately account for the relative accuracy of individual timing signals within the computations performed to determine position. In some circumstances, ranging or pseudoranging signals may be noisy due to atmospheric conditions, interference, or multipath effects, and such factors may impact certain signals more than others. Accordingly, there remains a need for improved N-lateration schemes that can accurately determine the position of a device even with noisy field data.